Over the last decade, smartphones have revolutionized the cellular phone industry. Text, image, and voice data, among other applications employed by smartphones and other devices have greatly increased the amount of traffic moving through cellular networks. Unfortunately, there is only a limited spectrum that can be used by cellular providers to serve their customers. Thus, spectrum bands are so packed today that their ownership has become an extremely expensive luxury that very few operators can afford. To improve network capacity, providers employed better modulation and coding techniques as well as advanced spectrum slicing techniques, which have led to a 25-fold gain in network capacity. The largest gains, a stunning 1600-fold, however, have come from spectrum reuse, originated by a reduction in the cell sizes and transmit distances.
Wireless service providers take advantage of spectrum reuse by deploying an increased number of base-stations with different coverage area extensions. Depending on their extension, cells can be classified—from largest to smallest by coverage area—into macro-cells, micro-cells, pico-cells, and femto-cells, representatively shown in FIG. 1. The first three types of cells fall into the category of operator deployed infrastructure. Distributed antennas—spatially separated antennas distributed over the macro-cell and connected to a macro-cell base station via a dedicated backhaul link—and relays—infrastructure devices with a wireless backhaul to the base station that forward calls and data to mobile devices—are also part of this technology group. The installation of the infrastructure in any of these cases must be carefully planned in order to optimize the performance of the network. This requires previous knowledge from the wireless service provider regarding the locations in which the network performance is experiencing coverage or throughput problems. Acquiring this knowledge, either reactively (performing measurements in response to user complaints) or proactively (performing measurements before receiving user complaints) represents increased costs for wireless service providers. In general, deploying base-stations and relays are expensive options for the wireless service provider, as their deployment involves planning, site, equipment, installation, energy, and maintenance costs.
Femto-cells are a new, promising technology that follow a different approach from the three cell types described above. Given that recent studies show that a great amount of voice and data services are provided or originated indoors, e.g. homes and buildings, it has become even more important to provide high throughput and coverage in those environments. Femto-cells aim at improving coverage and data rates in small indoor environments for a small number of users. Therefore, Femto-cell Access Points (“FAPs”) are acquired, owned, and installed by the final user. FAPs operate in the licensed spectrum, and the connection to the operator or service provider's core network is achieved via an IP-backhaul link, instead of through the provider's wireless access infrastructure. FIG. 2 shows the general scheme of a conventional femto-cell system.
From a user's perspective, femto-cells provide the benefits of 3G/4G data rates and high voice quality in indoor environments, with both increased battery life, and possibly lower call cost (as the wireless service providers may encourage femto-cell usage). From the wireless service provider's perspective, femto-cells also provide several benefits. Femto-cells represent a low-cost alternative to improve the coverage and throughput in indoor environments because the cost of the FAP can be transferred to the final user. By improving the user experience in indoor environments, wireless service providers are in a better position to compete with fixed providers of VoIP and WiFi. Another important benefit of femto-cells is that the traffic of the users that are served by the FAP is offloaded from the macro-cell through the wired IP-backhaul link, which reduces the traffic load within the operator's infrastructure and leaves more resources available to serve users that are not in a femto-cell layer.
While being advantageous over many prior systems, conventional femto-cells present many shortcomings, which severely limit their performance capabilities. Two of the major problems facing conventional femto-cells are (1) the interference caused by random femto-cell deployments, and (2) the incapability of guaranteeing acceptable Quality of Service (“QoS”) through the IP-backhaul link.
Interference Caused by Conventional Femto-cells Deployments
The characteristics of femto-cell deployments (within the coverage area of a single macro-cell base-station) lead to interference scenarios that can severely degrade the throughput of the femto-cell layer and the macro-cell layer. Because FAPs are acquired by the final user for an indoor environment, the coverage area of the femto-cell does not need to be large. In addition, the number of femto-cells within the coverage area of a macro-cell base-station can be quite large because each residence or building within the macro-cell can potentially have one or more femto-cells. These characteristics lead to a variety of interference cases, which are briefly described below.
Femto-cell to Macro-cell Interference: There are two primary classes of equipment within the macro-cell: (1) femto-cell user equipments (“fUEs”), which are devices within the femto-cell coverage area served by the femto-cell, i.e. data is routed between the fUEs and the core network through the IP-backhaul link; and (2) macro-cell user equipments (“mUEs”), which are devices within the macro-cell served by the base-station, i.e. data is routed between the core stations and the mUE's via the base-station. In the down-link (“DL”), the transmission from the FAP to the fUEs causes interference at the mUEs. In general, this interference increases, first, as the distance from the mUEs to the FAP decreases and, second, as the distance from the fUEs to the FAP increases. The second factor appears because the transmission power of the FAP (and the interference that it causes) increases as its distance to the fUEs increases. In the up-link (“UL”), the transmission from the fUEs to the FAP causes interference at the base station. This interference increases, first, as the distance from the fUEs to the base station decreases and, second, as the distance from the fUEs to the FAP increases. This second factor appears because the transmission power of the fUEs (and the interference they cause) increases as their distance to the FAP increases.
Macro-cell to Femto-cell Interference: In the DL, the transmission from the macro-cell base-station to the mUEs causes interference at the fUEs. In general, this interference increases as the distance from the fUEs to the base-station decreases. In the UL, the transmission from the mUEs to the base-station causes interference at the FAP. This interference increases, first, as the distance from the mUEs to the FAP decreases and, second, as the distance from the mUEs to the base-station increases. The second factor appears because the transmission power of mUEs increases as their distance to the base-station increases.
Femto-cell to Femto-cell Interference: Femto-cells also can interfere with other femto-cells, especially when the two femto-cells are geographically located close to each other. For example, consider two FAPs close to each other (e.g., in the same residential building), FAP1 and FAP2, serving fUE1 and fUE2, respectively. In the DL, the transmission from FAP1 to fUE1 causes interference at fUE2. In the same way, the transmission from FAP2 to fUE2 causes interference at fUE1. This interference increases as the distance between fUE2 and fUE1 decreases. Also, the interference increases as the distance from FAP1 to fUE1 increases (in the first case), and the distance from FAP2 to fUE2 increases (in the second case). This type of interference can severely degrade the performance of femto-cells in high density femto-cell deployments, such as residential buildings.
In a macro-cell layer, an mUE can choose to connect to the base-station that provides the “strongest” signal. In a network of femto-cells, however, mUEs usually will not be allowed to connect to the femto-cell (because the FAP and internet connection are paid for by the users of fUEs). This restriction further increases the severity of interference-related problems with femto-cells.
QoS Impairments in Conventional Femto-cells
In addition to the interference problems discussed above, conventional femto-cells also present many problems to users relating to the QoS they are capable of providing. In a typical femto-cell deployment, a wired link connects the FAP to the core network of the wireless service provider. In most cases, this wired link will be an internet connection provided by an Internet Service Provider (“ISP”). The performance of the internet connection is sensitive to network congestion, which can lead to packet loss, delay, and jitter. Therefore, the femto-cell data also will experience similar problems.
Congestion can occur at different levels of the network. At the Local Area Network (“LAN”) level (within the home or enterprise), congestion can occur due to multiple active devices sharing the network (e.g. laptops, desktops, game consoles, servers). At the ISP network level, congestion can occur due to a plurality of active clients sharing the ISP's network. At the “internet level,” which includes all the network devices that are not under the control of the ISP, congestion can occur due to a plurality of active users sending/receiving traffic across the network.
Conventional femto-cells employ many different techniques in an attempt to reduce congestion and improve QoS. For example, within the LAN, the final user could prioritize the data sent/received by the femto-cell to reduce the impact of congestion. If the ISP is aware of the QoS requirements of fUEs' data, the ISP could also prioritize data to reduce the impact of ISP network congestion in the QoS. Unfortunately, even if the ISP is able to perform the prioritization, congestion still can occur at the “internet level.” Thus, QoS problems persist. If, due to congestion problems, the QoS requirements of the fUEs' data are not satisfied, the final user perception will be that the femto-cell is not fulfilling its main purpose: high quality voice and data communications in indoor environments.
In addition to the above-mentioned shortcomings, providing synchronization and timing is another major problem with conventional femto-cells. Synchronization is needed to perform successful handovers, minimize multi-access interference, and ensure tolerable carrier offset. Obtaining accurate synchronization over the IP backhaul, however, can be very difficult. Further, the risk of call drops while a handover is performed (i.e., switched across from macro-cell to femto-cell or vice versa) is quite high.
Therefore, there is a desire for systems and methods that increase the capacity of a cellular network to serve a plurality of cellular devices while delivering high QoS data transfer with minimal interference. Various embodiments of the present invention provide such systems and methods.